Vertical meander inductor for small core voltage regulators

ABSTRACT

Vertical meander inductors for small core voltage regulators and approaches to fabricating vertical meander inductors for small core voltage regulators are described. For example, a semiconductor die includes a substrate. An integrated circuit is disposed on an active surface of the substrate. An inductor is coupled to the integrated circuit. The inductor is disposed conformal with an insulating layer disposed on an essentially planar surface of the substrate. The insulating layer has an undulating topography.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/319,429, filed on Jun. 30, 2014, which is a continuation of U.S.patent application Ser. No. 13/629,168, filed on Sep. 27, 2012, now U.S.Pat. No. 8,803,283, issued on Aug. 12, 2014, the entire contents ofwhich are hereby incorporated by reference herein.

TECHNICAL FIELD

Embodiments of the invention are in the field of on-die inductors and,in particular, vertical meander inductors for small core voltageregulators.

BACKGROUND

For the past several decades, the scaling of features in integratedcircuits has been a driving force behind an ever-growing semiconductorindustry. Scaling to smaller and smaller features enables increaseddensities of functional units on the limited real estate ofsemiconductor chips. For example, shrinking transistor size allows forthe incorporation of an increased number of memory devices on a chip,lending to the fabrication of products with increased capacity. Thedrive for ever-more capacity, however, is not without issue. Thenecessity to optimize the performance of each device becomesincreasingly significant.

On-die voltage regulation is designed to automatically maintain aconstant voltage level for an associated semiconductor die. A voltageregulator may be a simple “feed-forward” design or may include negativefeedback control loops. It may use an electromechanical mechanism, orelectronic components. Depending on the design, it may be used toregulate one or more AC or DC voltages.

Electronic components, such as inductors, may be implemented onsubstrates such as an integrated circuit die or a printed circuit board(PCB). Such implementations involve placing patterns of material (e.g.,as conductive material) on one or more substrate layers. This placementmay be through lithographic techniques. Inductors used for RFapplications in complementary metal oxide semiconductor (CMOS)technology are typically air-core spiral inductors. Various drawbacksare associated with these inductors. For instance, air-core spiralinductors typically require a substantial amount of space (area) on asubstrate (e.g., an IC die). Moreover, such inductors require ahigh-resistivity substrate.

Thus, significant improvements are still needed in the area of on-dieinductors for voltage regulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an angled view and corresponding cross-sectional viewof a strip-line inductor for on-die voltage regulation, in accordancewith an embodiment of the present invention.

FIG. 2A is an SEM image of a cross-sectional view of a portion of aninductor structure, in accordance with an embodiment of the presentinvention.

FIG. 2B is an SEM image of another cross-sectional view of a portion ofan inductor structure, in accordance with an embodiment of the presentinvention.

FIG. 2C is an SEM image of a top angled view of an inductor structure,in accordance with an embodiment of the present invention.

FIG. 3 illustrates scaling of a micro-strip or strip-line inductor froma total physical length of 800 microns to 650 microns by fabrication ofan inductor that meanders vertically, in accordance with an embodimentof the present invention.

FIGS. 4A and 4B illustrate cross-sectional views of operations in, andthe resulting structure of, a method of fabricating a meanderinginductor, in accordance with an embodiment of the present invention.

FIGS. 5A and 5B illustrate cross-sectional views of operations in, andthe resulting structure of, another method of fabricating a meanderinginductor, in accordance with an embodiment of the present invention.

FIG. 6 illustrates a computing device in accordance with oneimplementation of the invention.

DESCRIPTION OF THE EMBODIMENTS

Vertical meander inductors for small core voltage regulators andapproaches to fabricating vertical meander inductors for small corevoltage regulators are described. In the following description, numerousspecific details are set forth, such as specific integration andmaterial regimes, in order to provide a thorough understanding ofembodiments of the present invention. It will be apparent to one skilledin the art that embodiments of the present invention may be practicedwithout these specific details. In other instances, well-known features,such as integrated circuit design layouts, are not described in detailin order to not unnecessarily obscure embodiments of the presentinvention. Furthermore, it is to be understood that the variousembodiments shown in the Figures are illustrative representations andare not necessarily drawn to scale.

One or more embodiments described herein are targeted to the fabricationof an undulating inductor for, e.g., voltage regulation via an on-dievoltage regulator. Conventional voltage regulators may occupy asubstantial volume. For example, large ferrite inductors running at lowfrequency have provided previous solutions for voltage regulation byincluding the large ferrite inductors as mounted on a motherboard. Bycontrast, new approaches include incorporation of a voltage regulatorcloser in proximity to a microprocessor. Such approaches have associatedbenefits such as, but not limited to, reduced inductor volume, systemfactor gains from increased ferromagnetic, minimization of V_(droop), afast response time and small volume to enable granularity, and improvedoverall efficiency.

Accordingly, more recent approaches to voltage regulation for integratedcircuits involves moving the voltage regulator on-die to allow improvedvoltage regulation performance. To this end, micro-strip (strip-line)inductors are being investigated. Strip-line inductors may offer thelowest DC resistance option (since they travel in a straight line) andinherent coupling which allows higher currents prior to magneticsaturation. As an example, FIG. 1 illustrates an angled view andcorresponding cross-sectional view of a strip-line inductor for on-dievoltage regulation, in accordance with an embodiment of the presentinvention. Referring to FIG. 1, a strip line inductor 100 includesconductive wires 102 and 104 (such as copper wires). In an example, wire102 is an output winding for the inductor 100 and wire 104 is an inputwinding for inductor 100. The wires 102 and 104 are housed in a magneticmaterial 106 by an insulating layer 108. The inductor 100 has a height(h), a width (w), and a length (l). The magnetic material 106 has athickness (t_(m)) and a magnetic field direction (B) shown by the arrowsin the cross-sectional view of FIG. 1. Thus, the inductor 100 is amagnetic core inductor as opposed to an air core inductor.

FIGS. 2A-2C are scanning electron microscope (SEM) images of variousviews of a strip-line inductor, in accordance with an embodiment of thepresent invention. FIG. 2A is an SEM image 200A of a cross-sectionalview of a portion of an inductor structure 200. Referring to FIG. 2A,first and second wires 202 and 204, such as output winding and inputwinding wires are housed in a magnetic dome 210, above a magneticmaterial 206. FIG. 2B is an SEM image 200B of another cross-sectionalview of a portion of the inductor structure 200. Referring to FIG. 2B, amagnetic via 212 couples the magnetic dome 210 with the underlyingmagnetic material 206 to surround the wires 202 and 204 (wires not shownin this view). FIG. 2C is an SEM image of a top angled view of theinductor structure 200. Referring to FIG. 2C, more than one inductor mayshare the same magnetic material, such as inductors #1 and #2, depictedin FIG. 2C. Also seen is the top of magnetic dome 210.

Since an inductor structure is a key component in modern voltageregulator circuits based on buck or hybrid circuit topologies, theinductor structure needs to occupy less volume or real estate yetwithout impacting performance of the inductor structure, as componentsscale. To maximize efficiency during average and peak current loads, itmay be necessary that the voltage regulator inductor area be equal to orless than the microprocessor core area. As such, currently there is noknown inductor-based voltage regulator solution that will fit inside anXY layout area of future core products based on scaling of today'sdimensions. An issue is that as the core (and other components such asgraphics) shrink they may require more turbo current density (eventhough the overall power is dropping). Current materials for planarstraight inductors may only be useful for the next generation or so,otherwise new material or multi-level inductors may be needed ifplanarity of the inductors is maintained. Although material solutionsmay address some efficiency gains, there may not be sufficientinductance (L) achieved upon shrinking of the associated inductor.Nonetheless, voltage regulation scaling may be critical to futureproducts such as system-on-chip (SoC) components.

Thus, in an embodiment, the above described micro-strip or strip-lineinductors may be further scaled to accommodate scaling in thesemiconductor industry. For example, in accordance with an embodiment ofthe present invention, the wire strips (typically flat and straight) areprovided with a meandering or undulating third dimension to provide alonger strip relative to a give planar surface dimension. As such,inductance is increased per XY area by shrinking area when the inductormeanders vertically. Such an approach may be used regardless of eventuallocation of the inductors, including the possibility to process on awafer or substrate backside. Such embodiments may enable scaling ofmodern devices in volume, which will likely be accommodated by scalingof the voltage regulator as well.

As an example, FIG. 3 illustrates scaling of a micro-strip or strip-lineinductor from a total physical length of 800 microns to 650 microns byfabrication of an inductor that meanders vertically, in accordance withan embodiment of the present invention. Referring to FIG. 3, a planar orflat strip-line inductor 300 is shown to run a length of approximately800 microns. Beyond thickness, the planar or flat strip-line inductor300 has no dimension vertically, e.g., in the z-direction. By contrast,a meandering (or undulating) strip-line inductor 350 has a length of 800microns, but only occupies approximately 650 microns in actual length.The meandering or undulating inductor has a dimension vertically (e.g.,beyond mere thickness in the z-direction). For example, the inductorstructure 350 meanders or undulates in the vertical direction byapproximately 40 microns. This approach effectively accomplishes ashrink of the actual length (e.g., 650 microns instead of 800 microns)needed to accommodate the inductor 350 without actually reducing thelength (e.g., 800 microns) of the inductor 350.

In a first example, FIGS. 4A and 4B illustrate cross-sectional views ofoperations in, and the resulting structure of, a method of fabricating ameandering inductor, in accordance with an embodiment of the presentinvention.

Referring to FIG. 4A, an insulating layer 402, such as a layer ofsilicon dioxide, is formed above a substrate 400, such as a siliconsubstrate of a semiconductor die. The insulating layer 402 is formed tohave an undulating topography. In an embodiment, the undulatingtopography of the insulating layer 402 is formed by reflow of alithographically patterned insulating layer. In another embodiment, theundulating topography of the insulating layer 402 is formed by usinggrayscale lithography.

In the latter case, using grayscale lithography, three-dimensionalmicrostructures are formed in a thick layer of photoresist. The fastestand easiest way to perform this is with a maskless laser lithographysystem, which uses an intensity modulated laser beam to write thestructures directly into the resist. The resist is applied to theinsulating layer of the substrate by spin- or spray-coating and can beseveral hundred microns thick, but most applications only require athickness of less than 50 microns. Using data from an electronic design,the laser lithography system exposes the resist with a variable dose andthe three-dimensional structures remain in the resist after thedevelopment process. The final result of this entire process depends onthe electronic data, the laser writing and the development, and all ofthese operations can be independently tuned to produce the best result.The optical structures in the photoresist can subsequently betransferred into insulating layer on the substrate by reactive ionetching. In another embodiment, however, gray tone masks are used topattern the insulating layer to have an undulating topography.

Referring to FIG. 4B, a meandering inductor structure 404, such as ameandering version of the strip-line inductors described in associationwith FIGS. 1 and 2A-2C is formed above the insulating layer 402 havingthe undulating topography. Thus, in an embodiment, a semiconductor die450 includes a substrate 400. An integrated circuit (not shown) isdisposed on an active surface (452 or 454) of the substrate 400. Theinductor 404 is coupled to the integrated circuit. The inductor 404 isdisposed conformal with the insulating layer 402 disposed on anessentially planar surface of the substrate 400. However, the insulatinglayer 402 has an undulating topography.

In one such embodiment, the insulating layer 402 having the undulatingtopography is disposed above the active surface of the substrate 400,above the integrated circuit, in which case in which case the integratedcircuit is formed on active surface 454. In another such embodiment, theinsulating layer 402 having the undulating topography is disposed on aback surface of the substrate 400, in which case the integrated circuitis formed on active surface 452. In a specific embodiment of the latterarrangement, the inductor 404 is coupled to the integrated circuit onactive surface 452 by one or more through silicon vias disposed insubstrate 400 (not shown).

In a second example, FIGS. 5A and 5B illustrate cross-sectional views ofoperations in, and the resulting structure of, another method offabricating a meandering inductor, in accordance with an embodiment ofthe present invention.

Referring to FIG. 5A, a substrate 500, such as a silicon substrate of asemiconductor die, is formed to have an undulating topography. In anembodiment (as shown), an insulating layer 502, such as a layer ofsilicon dioxide, is formed above the substrate 500, conformal with theundulating topography. In one embodiment, the undulating topography isfaceted, as depicted in FIG. 4A. However, in other embodiments, theundulating topography of the substrate 500 is not faceted.

In an embodiment, the undulating topography of the substrate 500 isformed by etching a silicon substrate. In one such embodiment, isotropicetching is used which involves an etching process which progresses atthe same speed in all directions. In another embodiment, long and narrowholes in a mask are used to produce v-shaped grooves in the siliconsubstrate. The surface of these grooves can be atomically smooth if theetch is carried out correctly, with dimensions and angles beingextremely accurate. In another embodiment, anisotropic etching is used.For example, some single crystal materials, such as silicon, havedifferent etching rates depending on the crystallographic orientation ofthe substrate. This is known as anisotropic etching and one of the mostcommon examples is the etching of silicon in KOH (potassium hydroxide),where Si <111> planes etch approximately 100 times slower than otherplanes (crystallographic orientations). Therefore, etching a rectangularhole in a (100)-Si wafer results in a pyramid shaped etch pit with 54.7°walls, instead of a hole with curved sidewalls as with isotropicetching. In other embodiments, dry etching, such as vapor or plasmaetching is used.

Referring to FIG. 5B, a meandering inductor structure 504, such as ameandering version of the strip-line inductors described in associationwith FIGS. 1 and 2A-2C is formed above the surface of the substratehaving the undulating topography. Thus, in an embodiment, asemiconductor die 550 includes a substrate 500. An integrated circuit(not shown) is disposed on an active surface 552 of the substrate 500.The inductor 504 is coupled to the integrated circuit. The inductor 504is disposed conformal with a back surface 554 of the substrate 500, theback surface having the undulating topography.

In one such embodiment, the inductor is disposed on an insulating layer502 disposed on the back surface 554 of the substrate 500. Theinsulating layer 502 is conformal with the back surface 554 having theundulating topography, as depicted in FIG. 5B. In an embodiment, theinductor 504 is coupled to the integrated circuit on active surface 552by one or more through silicon vias disposed in substrate 500 (notshown).

More generally, various embodiments described herein may be directed totechniques involving electronic components fabrication. For instance, insome embodiments, an apparatus may include a first magnetic layer, asecond magnetic layer, and a conductive pattern. The conductive patternis at a third layer between the first and second magnetic layers, and iscomposed of a metal such as copper or aluminum. Moreover, one or moremagnetic vias connect the first and second magnetic layers. In someembodiments, the magnetic layers and vias may operate as ferromagneticcores or shields, e.g., and may be composed of materials that can beformed to have ferromagnetism such as Co, Ni, or iron oxide.Furthermore, the magnetic layers and vias may be integrated on a chip(e.g., for on-die magnetics). Also, they may be implemented with CMOStechnology or processes or, alternatively, on lower cost panel levelprocessing. The apparatus may be included in inductors, transformers,transmission lines, RF circuits, wireless applications, voltageregulators and so forth.

As described herein, embodiments may advantageously provide inductors ofcomparable or better performance than current approaches, and that havea much smaller footprint. Furthermore, some embodiments enable avoidanceof the blockage of space underneath inductors. Also, some embodimentsmay be implemented with low-resistivity substrates. This allows, forexample, co-integration of digital and RF circuits using a high-speedCMOS process.

Moreover, embodiments may provide inductors that may be used at lowerfrequencies. Exemplary lower frequency applications may includeswitching amplifiers used as envelope generators for high-efficiency RFpower amplifiers. Such applications may involve modulation schemesrequiring class-A linear RF power amplifiers, which have a theoreticalefficiency of less than 12.5%. Other applications include resonant gatedrivers for high-power DC-DC converters, low to mid-power on-die DC-DCconverters. Furthermore, inductors described herein may be an integralcollateral for system-on-chip (SoC) designers.

FIG. 6 illustrates a computing device 600 in accordance with oneimplementation of the invention. The computing device 600 houses a board602. The board 602 may include a number of components, including but notlimited to a processor 604 and at least one communication chip 606. Theprocessor 604 is physically and electrically coupled to the board 602.In some implementations the at least one communication chip 606 is alsophysically and electrically coupled to the board 602. In furtherimplementations, the communication chip 606 is part of the processor604.

Depending on its applications, computing device 600 may include othercomponents that may or may not be physically and electrically coupled tothe board 602. These other components include, but are not limited to,volatile memory (e.g., DRAM), non-volatile memory (e.g., ROM), flashmemory, a graphics processor, a digital signal processor, a cryptoprocessor, a chipset, an antenna, a display, a touchscreen display, atouchscreen controller, a battery, an audio codec, a video codec, apower amplifier, a global positioning system (GPS) device, a compass, anaccelerometer, a gyroscope, a speaker, a camera, and a mass storagedevice (such as hard disk drive, compact disk (CD), digital versatiledisk (DVD), and so forth).

The communication chip 606 enables wireless communications for thetransfer of data to and from the computing device 600. The term“wireless” and its derivatives may be used to describe circuits,devices, systems, methods, techniques, communications channels, etc.,that may communicate data through the use of modulated electromagneticradiation through a non-solid medium. The term does not imply that theassociated devices do not contain any wires, although in someembodiments they might not. The communication chip 606 may implement anyof a number of wireless standards or protocols, including but notlimited to Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE,GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, derivatives thereof, as well asany other wireless protocols that are designated as 3G, 4G, 5G, andbeyond. The computing device 600 may include a plurality ofcommunication chips 606. For instance, a first communication chip 606may be dedicated to shorter range wireless communications such as Wi-Fiand Bluetooth and a second communication chip 606 may be dedicated tolonger range wireless communications such as GPS, EDGE, GPRS, CDMA,WiMAX, LTE, Ev-DO, and others.

The processor 604 of the computing device 600 includes an integratedcircuit die packaged within the processor 604. In some implementationsof the invention, the integrated circuit die of the processor includesone or more passive devices, such as on-die inductors built inaccordance with implementations of the invention. The term “processor”may refer to any device or portion of a device that processes electronicdata from registers and/or memory to transform that electronic data intoother electronic data that may be stored in registers and/or memory.

The communication chip 606 also includes an integrated circuit diepackaged within the communication chip 606. In accordance with anotherimplementation of the invention, the integrated circuit die of thecommunication chip includes one or more passive devices, such as on-dieinductors built in accordance with implementations of the invention.

In further implementations, another component housed within thecomputing device 600 may contain an integrated circuit die that includesone or more passive devices, such as on-die inductors built inaccordance with implementations of the invention.

In various implementations, the computing device 600 may be a laptop, anetbook, a notebook, an ultrabook, a smartphone, a tablet, a personaldigital assistant (PDA), an ultra mobile PC, a mobile phone, a desktopcomputer, a server, a printer, a scanner, a monitor, a set-top box, anentertainment control unit, a digital camera, a portable music player,or a digital video recorder. In further implementations, the computingdevice 600 may be any other electronic device that processes data.

The specific implementations described above are not meant to belimiting to the spirit and scope of embodiments of the presentinvention. For example, in another embodiment, a voltage regulator (VR)circuit is included as a separate chip—often referred to as a powermanagement integrated circuit (PMIC) chip. Another embodiments includesusing of surface mount meandering inductors on a package, yet usingtransistors of an associated IC for switches. Another embodimentincludes surface mount of an entire VR on a package or embedded in apackage.

Thus, embodiments of the present invention include vertical meanderinductors for small core voltage regulators and approaches tofabricating vertical meander inductors for small core voltageregulators.

In an embodiment, a semiconductor die includes a substrate. Anintegrated circuit is disposed on an active surface of the substrate. Aninductor is coupled to the integrated circuit. The inductor is disposedconformal with an insulating layer disposed on an essentially planarsurface of the substrate. The insulating layer has an undulatingtopography.

In one embodiment, the insulating layer having the undulating topographyis disposed on a back surface of the substrate.

In one embodiment, the inductor is coupled to the integrated circuit byone or more through silicon vias (TSVs) disposed in the substrate.

In one embodiment, the insulating layer having the undulating topographyis disposed above the active surface of the substrate, above theintegrated circuit.

In one embodiment, the inductor is a magnetic core inductor.

In one embodiment, the inductor is included in an on-die voltageregulator.

In an embodiment, a semiconductor die includes a substrate. Anintegrated circuit is disposed on an active surface of the substrate. Aninductor is coupled to the integrated circuit. The inductor is disposedconformal with a back surface of the substrate, the back surface havingan undulating topography.

In one embodiment, the inductor is disposed on an insulating layerdisposed on the back surface of the substrate. The insulating layer isconformal with the back surface having the undulating topography.

In one embodiment, the inductor is coupled to the integrated circuit byone or more through silicon vias (TSVs) disposed in the substrate.

In one embodiment, the undulating topography of the back surface isfaceted.

In one embodiment, the undulating topography of the back surface isnon-faceted.

In one embodiment, the inductor is a magnetic core inductor.

In one embodiment, the inductor is included in an on-die voltageregulator.

In an embodiment, a method of fabricating a semiconductor structureincludes forming an integrated circuit on an active surface of asubstrate. The method also includes forming an insulating layer on anessentially planar surface of the substrate, the insulating layer havingan undulating topography. The method also includes forming an inductorcoupled to the integrated circuit, the inductor formed conformal withthe insulating layer.

In one embodiment, forming the insulating layer having the undulatingtopography includes reflowing a lithographically patterned insulatinglayer.

In one embodiment, forming the insulating layer having the undulatingtopography includes using grayscale lithography.

In one embodiment, forming the insulating layer having the undulatingtopography includes forming the insulating layer on a back surface ofthe substrate.

In one embodiment, forming the inductor coupled to the integratedcircuit includes coupling the inductor to the integrated circuit by oneor more through silicon vias (TSVs) formed in the substrate.

In one embodiment, forming the insulating layer having the undulatingtopography includes forming the insulating layer above the activesurface of the substrate, above the integrated circuit.

In one embodiment, forming the inductor includes forming a magnetic coreinductor.

In one embodiment, forming the inductor includes forming an on-dievoltage regulator.

In an embodiment, a method of fabricating a semiconductor structureincludes forming an integrated circuit on an active surface of asubstrate. The method also includes forming an undulating topography ona back surface of the substrate. The method also includes forming aninductor coupled to the integrated circuit, the inductor formedconformal with the back surface of the substrate.

In one embodiment, forming the undulating topography on the back surfaceof the substrate includes using an isotropic etch of a siliconsubstrate.

In one embodiment, forming the undulating topography on the back surfaceof the substrate includes using an isotropic etch of a siliconsubstrate.

In one embodiment, the method further includes, prior to forming theinductor, forming an insulating layer conformal with the back surfacehaving the undulating topography. The inductor is formed on theinsulating layer formed on the back surface of the substrate.

In one embodiment, forming the inductor coupled to the integratedcircuit includes coupling the inductor to the integrated circuit by oneor more through silicon vias (TSVs) formed in the substrate.

In one embodiment, forming the undulating topography includes forming afaceted topography.

In one embodiment, forming the undulating topography includes forming anon-faceted topography.

In one embodiment, forming the inductor includes forming a magnetic coreinductor.

In one embodiment, forming the inductor includes forming an on-dievoltage regulator.

What is claimed is:
 1. A semiconductor die, comprising: a substrate having an active front surface and a back surface; an integrated circuit disposed on the active front surface of the substrate; and an inductor disposed above the back surface of the substrate and coupled to the integrated circuit, wherein both an uppermost surface and a lowermost surface of the inductor have an undulating topography based on centrally rounded protrusions.
 2. The semiconductor die of claim 1, wherein the inductor is coupled to the integrated circuit by one or more through silicon vias (TSVs) disposed through the substrate.
 3. The semiconductor die of claim 1, wherein the inductor is a magnetic core inductor.
 4. The semiconductor die of claim 1, wherein the inductor is included in an on-die voltage regulator.
 5. A method of fabricating a semiconductor structure, the method comprising: forming an integrated circuit on an active front surface of a substrate; and forming an inductor above a back surface of the substrate and coupled to the integrated circuit, wherein both an uppermost surface and a lowermost surface of the inductor have an undulating topography based on centrally rounded protrusions.
 6. The method of claim 5, wherein forming the inductor coupled to the integrated circuit comprises coupling the inductor to the integrated circuit by one or more through silicon vias (TSVs) formed through the substrate.
 7. The method of claim 5, wherein forming the inductor comprises forming a magnetic core inductor.
 8. The method of claim 5, wherein forming the inductor comprises forming an on-die voltage regulator.
 9. A method of fabricating a semiconductor structure, the method comprising: providing a substrate having an integrated circuit on an active front surface of the substrate; and forming an inductor above a back surface of the substrate and coupled to the integrated circuit, wherein both an uppermost surface and a lowermost surface of the inductor have an undulating topography based on centrally rounded protrusions.
 10. The method of claim 9, wherein forming the inductor coupled to the integrated circuit comprises coupling the inductor to the integrated circuit by one or more through silicon vias (TSVs) formed through the substrate.
 11. The method of claim 9, wherein forming the inductor comprises forming a magnetic core inductor.
 12. The method of claim 9, wherein forming the inductor comprises forming an on-die voltage regulator. 